The determination of true surface temperature of Silicon (Si) during the various processes encountered in the fabrication of a microelectronic chip is an important parameter that determines the quality of the manufactured product. One of the methods used in determining the surface temperature is based on remotely sensing the radiation from the silicon surface. If the surface emissivity is known, one can deduce the surface temperature from the remote measurement. However the surface emissivity is usually not known accurately enough, and moreover it is a function of temperature and the surface treatment the silicon is subjected to.
The total radiation from a surface is composed of a self-emitted part and a reflected part. At high temperatures the self-emitted radiation from a Si surface at a given viewing angle has a certain degree of linear polarization as determined by its complex refractive index. If in addition a known source of radiation is reflected from the surface, then this reflected component has a degree of polarization (DOP) that is in opposition to that of the emitted component. As shown previously in an article by one of the present inventors, Balfour L. S. (Leslie Salem), “Infrared polarization thermometry using an imaging radiometer”, QIRT 94, Eurotherm Series 42-EETI ed., Paris 1995 pp. 103-105, one can obtain a null value for the DOP when the sources of the reflected and emitted components are at the same temperature. This forms the basis for an accurate surface temperature determination, without the need to know the surface emissivity.
The method described in the QIRT 94 reference above measured a painted copper surface in ordinary atmospheric surroundings, which affect the heat loss processes occuring on the surface. In contrast, silicon-processing systems are typically vacuum systems, in which both the access to the measured surface and the heat transfer between the Si surface and the surroundings are more complicated. In the QIRT94 reference, no consideration was given to the rate of change of temperatures, neither of the source nor of the examined surface. All measurements were carried out under steady state conditions, which are irrelevant to measurements of silicon surfaces in which there are rapid variations in the surface temperature, e.g. in Rapid Thermal Processing (RTP). RTP is characterized by two different aspects: fast heating rates (up to a few hundred degrees Celsius per second) and short holding times at a constant temperature (down to 1 second per process). Consequently, there are no known instances in which the method described in the QIRT reference was applied to temperature measurements of silicon surfaces, probably due to the inherent difficulties and complications involved.
The use of reflected linearly polarized light for real time, Si surface temperature detection is known, as disclosed for example in U.S. Pat. No. 5,313,044 to Massoud et al. However, the method disclosed therein suffers from a number of disdvantages, including a requirement for a-priori knowledge of the temperature dependence of the Si wafer refractive index. This temperature dependence is derived under steady state conditions, in which the silicon wafer is at equilibrium with its surroundings at elevated temperatures. In typical silicon thermal processing conditions, and in particular under RTP, the wafer is far from thermodynamic equilibrium. It is safe to assume that under such conditions the refractive index versus temperature curve is different from that in steady state. This can lead to major errors in wafer temperature estimation.
Hence, a remote temperature measurement system and method for silicon thermal processing, particularly RTP, that requires no a-priori knowledge of parameters such as described above, and is independent of emissivity, is highly desirable.